The present invention relates to a photochromic lens having the lighting control function.
Photochromic lenses are known having a lighting control function utilizing a photochromic compound, which is capable of reversible isomerization between two states in which the absorption spectra are different from each other, under the influence of light and without changing the molecular weight thereof.
Some known photochromic lenses are made of glass, while others are made of a resin. Some of such resin photochromic lenses have a photochromic coating surface layer formed on the lens substrate, and others include a resin lens substrate in which a photochromic compound is dispersed.
Known photochromic compounds used in such resin photochromic lenses include spirooxazine compounds and tetra- (or hexa-)benzoperopyrene compounds.
Of these photochromic compounds, spirooxazine compounds show a tendency that their weather resistance decreases when exposed to ultraviolet rays of short wavelengths. Thus a spirooxazine compound is used in the form of particulates enclosed in light-shielding inorganic films and dispersed in a resin matrix (JP Patent Publication 63-175071A; Patent document 1).
An optical member is known which includes a polarizing film and a coating layer or layers covering one or both sides of the polarizing film, the coating layer or layers being made of a polyurethane material containing 0.05 to 2.0% by weight of a spirooxazine compound as a lighting control (photochromic) compound (U.S. Pat. No. 7,035,010; Patent document 2).
A coating solution composition for lighting control plastic lenses is known which contains a spirooxazine compound which changes color by light and which is dissolved in toluene or ethylene chloride (JP Patent Publication 2006-502423, paragraph 0016; Patent document 3).
There are only two known methods for mixing a conventional spirooxazine compound in resin, which are: enclosing particulates of a spirooxazine compound in light-shielding inorganic films and dispersing them in a resin matrix (Patent document 1); and adding a spirooxazine compound to a polyurethane resin (Patent document 2). Hexane, xylene, toluene, methylene chloride and ethyl acetate are the only organic solvents that are known to be suitable for use with a spirooxazine compound (Patent document 3).
None of the methods disclosed in these prior art references can sufficiently exhibit the photochromic properties of a spirooxazine compound. In particular, with these methods, the spirooxazine compound is not colored sufficiently when exposed to strong ultraviolet radiation in a bright environment, and also, a long time is needed until the color disappears when the compound is thereafter placed in a dark place where the ultraviolet radiation is low. In particular, it takes about 15 minutes until the color disappears completely.
The longer it takes until the color disappears, the more remarkable the variation in color tone if a plurality of photochromic pigments are used, because the speeds at which the respective pigments lose colors are different. This means that the color tone in a bright place is markedly different from the color tone in a dark place.
If a lens is placed in an environment where the lens is exposed to a high dose of ultraviolet radiation, it is known that the lens deteriorates to the depth of about 0.5 mm. Photochromic eyeglasses are known of which the surface layers of the lenses are coated with or impregnated with a photochromic compound (and not with an ultraviolet absorbent).
One problem with this type of eyeglass lenses is the short duration of their photochromic properties. That is, ordinarily, such lenses completely lose their photochromic properties in two years.
If particles of a photochromic compound are wrapped in inorganic films which can reflect ultraviolet radiation and dispersed in a resin matrix in order to improve the weather resistance of the photochromic compound, the films tend to interfere with the ability of the photochromic pigment to reveal and lose its color.
If a photochromic compound is used in such a manner that the photochromic compound cannot sufficiently exhibit its photochromic properties, it is necessary to use an increased amount of this compound to obtain sufficient photochromic properties. This slows down the speed with which the photochromic compound can lose its color.